As papermaking materials have typically been used natural cellulose fibers, rayon fibers and vinylon fibers. These conventional materials are however being now superseded by polyester fibers in some quarters of the papermaking industry for the reduction of production costs and to meet the intensifying requirements for the performance quality of paper and paper products. One of the reasons for this is the superiority of the polyester fibers over the conventional papermaking materials in, for example, mechanical and electrical properties, resistance to heat, dimensional stability and hydrophobic properties. Polyester fibers are thus considered to take the place of the conventional papermaking materials at an accelerated rate from now on, to keep pace with the growing requirements for higher quality as the industrial structure advances.
Used as the papermaking materials are short polyester fibers which are in most cases manufactured for use as textile materials. Such polyester fibers are thus added with one or more of an anionic surfactant such as a potassium laurylphosphate, a nonionic surfactant such as a fatty acid alcohol with an additive of ethylene oxide, and a cationic surfactant such as a quaternary ammonium salt. This is intended principally to improve the passability of the fibers through a carding machine, the antistatic property and the sliver-forming ability of the fibers and to reduce the roller wrap-up tendency of the polyester fibers. Addition of these surfactant compounds is however practically useless for the purpose of improving the dispersibility of the fibers or, if they are of any use at all, the degree of usefulness is only quite limited. Extreme difficulties are encountered when relatively long polyester fibers with relatively small denier numbers in particular are to be dispersed uniformly.
Known polyester fibers for use as papermaking materials for this reason inevitably have extremely low degrees of dispersibility, which require the fibers to be processed with extremely low degrees of fiber density during a paparmaking process. This is reflected by an extremely low production efficiency of papermaking with use of the conventional polyester fibers.
To provide a solution to this problem, we have proposed papermaking polyester fibers having a particular polyesterpolyether block copolymer deposited on the surfaces of the fibers, such polyester fibers being disclosed in Japanese Provisional Patent Publication No. 58-208500. The polyester fibers excel in dispersibility and have relatively high degrees of smoothness. Such high degrees of smoothness of the polyester fibers create difficulties when the fibers are to be packed, especially when they are relatively short and are to be packed in the baling box of a box-type staple baling machine. When the door of the baling box of the machine is opened up with the short polyester fibers stuffed and compacted in the box, fragments of the bale formed in the box may slip down out of the baling box and thus hinder the smooth stream of the baling operation. An excess of smoothness of the polyester fibers is further responsible for inadequate degrees of various mechanical strengths, such as tenacity, of the paper prepared from such fibers.
The present invention contemplates elimination of all these drawbacks of known polyester fibers used typically as papermaking materials and it is accordingly an important object of the present invention to provide polyester fibers with an increased degree of dispersibility and a reduced degree of smoothness. Another important object of the present invention is to provide polyester fibers having improved adaptability to papermaking materials and useful for the production of paper with increased degrees of mechanical strengths and of texture. Still another object of the invention is to provide polyester fibers which can be packed with ease and at an increased efficiency. The performance efficiency in packing or baling fibers will be herein referred to as "packing work performance" of the fibers.